The present invention relates to an ammonia fuel cell, especially to a fuel cell that directly utilizes ammonia as a fuel without prior treatment to decompose the ammonia and remove traces of undecomposed ammonia. This fuel cell produces electrical energy based on the partial oxidation reaction
            NH      3        +                  3        4            ⁢              O        2              →                    1        2            ⁢              N        2              +                  3        2            ⁢              H        2            ⁢              O        .            
A polymer exchange membrane (PEM) H2/O2 fuel cell is an example of a fuel cell for widespread commercial application. However, highly pure hydrogen must be delivered to a PEM H2/O2 fuel cell. Sources of hydrogen with high gravimetric and volumetric density are also needed. This need is most acute for mobile applications. Physical storage as compressed gas or liquid can achieve gravimetric densities of approximately 10 weight percent. Both compressing and liquefying hydrogen are technologically difficult and use up a sizeable fraction (by up to approximately 30%) of the stored hydrogen's energy. Chemical storage as metal hydrides can achieve only 1–8 weight percent. Ammonia contains approximately 17 weight percent hydrogen. Ammonia is therefore attractive as a hydrogen source for PEM H2/O2 fuel cells. Hydrogen can be obtained from ammonia separate from the fuel cell via the decomposition reaction
      NH    3    →                    1        2            ⁢              N        2              +                  3        2            ⁢                        H          2                .            
This reaction is endothermic and consumes approximately 13% of the energy in the ammonia. However, this reaction requires high temperatures of 400–1000° C. PEM H2/O2 fuel cells typically run at 80° C. The difference between the ammonia decomposition temperature and the fuel cell temperature leads to inefficiencies. In practice, up to 40% of the energy in the ammonia can be used for the decomposition, which is significantly higher than the 13% theoretical loss noted above. Moreover, ammonia cannot be used directly in a PEM fuel cell because the ammonia would not decompose at the temperatures that PEM fuel cells operate and the undecomposed ammonia would poison the fuel cell catalyst.
Ammonia decomposition reactors have been developed to catalytically decompose ammonia into N2+H2. The N2+H2 mixture is subsequently fed into the anode of a H2/O2 fuel cell. However, a practical disadvantage is that any residual ammonia will poison the anode of a PEM fuel cell. Residual ammonia can result from an incomplete reaction or, depending on the decomposition temperature, from the decomposition equilibrium. Another disadvantage is that ammonia decomposition is endothermic and so energy must be continually supplied to a decomposition reactor to keep it at the required temperature. This results in a loss in efficiency for the fuel processing/fuel cell system as a whole.
U.S. Pat. Nos. 5,055,282 and 5,976,723 disclose a method for cracking ammonia into hydrogen and nitrogen, comprising exposing an ammonia cracking catalyst to ammonia under conditions effective to produce nitrogen and hydrogen, wherein said ammonia cracking catalyst contains an alloy having the general formula Zr1−xTixM1M2, wherein M1 and M2 are selected independently from the group consisting of Cr, Mn, Fe, Co, and Ni, and x is in the range from 0.0 to 1.0 inclusive, and between about 20% by weight and about 50% by weight of Al. The disclosures of U.S. Pat. Nos. 5,055,282 and 5,976,723 are hereby incorporated herein by reference.
U.S. Patent Application No. 2002/0021995 discloses an apparatus and method for decomposing NH3. A fluid containing NH3 is passed in contact with a tubular membrane that is a homogeneous mixture of a ceramic and a first metal, with the ceramic being selected from one or more of a cerate having the formula of M′Ce1−xMx″O3−δ, zirconates having the formula M′Zr1−xMx″O3−δ, stannates having the formula M′Sn1−xMx″O3−δ, where M′ is a group IIA (Be, Mg, Ca, Sr, Ba, Ra) metal, M″ is a dopant metal of one or more of Ca, Y, Yb, In, Nd, Gd or mixtures thereof and δ is a variable depending on the concentration of dopant and is in the range of from 0.001 to 0.5, the first metal is an element selected from the group consisting of Pt, Ag, Pd, Fe, Co, Cr, Mn, V, Ni, Au, Cu, Rh, Ru, Os, Ir and mixtures thereof. The tubular membrane has a catalytic metal on the side thereof in contact with the fluid containing NH3 which is effective to cause NH3 to decompose to N2 and H2. When the H2 contacts the membrane H+ ions are formed which pass through the membrane driving the NH3 decomposition toward completion. What is needed is an ammonia fuel cell for generating electrical energy.